Conductive polysilicon thin films are used in a wide variety of microelectronic devices. In many applications, these conductive polysilicon thin films should have a uniform thickness and a flat surface, because these factors may influence the performance of the semiconductor devices which include the conductive polysilicon thin film.
Electrodes such as, for example, gate electrodes, of semiconductor devices are generally formed via chemical vapor deposition (CVD), low pressure CVD (LPCVD), plasma-enhanced CVD (PECVD) or epitaxial growth techniques. For example, the gate electrodes of conventional flash memory devices are generally formed using one of the above-listed processes. When CVD is used to form a polysilicon film, superior step coverage can be obtained at a low temperature. However, as semiconductor devices become highly integrated, the number of elements in the device, and hence the number of step gaps between elements, becomes large. It can be difficult to fill these large step differences using, CVD. In addition, when the pattern density is varied, the thickness of the polysilicon film can become uneven because of a pattern loading effect, which can increase the roughness of the surface of the polysilicon film.
Electrodes of conventional semiconductor devices are typically formed by depositing a polysilicon film and then performing various etching and cleaning processes on the deposited polysilicon layer. These etching and cleaning processes may remove a significant amount of the deposited polysilicon. If the surface of the polysilicon film is rough, the etching and cleaning processes may tend to further exacerbate the variations in thickness. In some cases, pitting can result when a large amount of polysilicon is removed from a hollow portion of the rough surface. In order to reduce surface roughness, the surface of the polysilicon film may be processed with O3. However, if the polysilicon film is thinner than about 140 Å, the surface roughness may not be improved by such an O3 process.
FIG. 1 is a graph illustrating the thickness of a polysilicon film (and the root mean squares (Rms) surface roughness thereof measured (A) after the polysilicon film is deposited using LPCVD, (B) after a wet-etching process using diluted HF (pure water: HF=200:1) which is performed to remove an oxide layer using the polysilicon layer as the etching mask, and (C) after a dry-etching process which is performed to remove an oxide layer using the polysilicon film as the etching mask. As shown in column (A) of FIG. 1, the Rms of the polysilicon film is about 150 Å as deposited. Thus, the surface roughness of the polysilicon film is comparatively high after deposition. As shown in columns (B) and (C) of FIG. 1, the thickness of the polysilicon film is reduced and the surface roughness is increased during the subsequent wet and dry etching processes because a significant amount of the polysilicon may be removed while removing peripheral layers. Accordingly, the polysilicon film is typically deposited to be much thicker than the target thickness of the polysilicon film. Furthermore, the initial thickness of the polysilicon film is typically selected to be sufficient to avoid pitting during subsequent processing steps. Therefore, the initial deposition thickness of the polysilicon film may be much thicker than the target thickness.
As described above, using conventional methods of forming polysilicon films it can be difficult to produce layers having uniform thickness and low surface roughness. Epitaxial growth techniques may be used to overcome these limitations, but this requires a high vacuum process at a pressure of about 10−6 Torr or less.